The use of poles are well known for such applications as for carrying utility power lines and the like, for accommodating the placement of lights thereon, or for accommodating the placement of other devices thereon a desired distance from the ground. Such poles have been conventionally formed from solid wood, steel, aluminum, or concrete having a desired thickness or outside diameter, and have also been formed from metal having an inside and outside diameter designed to provide a desired wall thickness.
A key factor to consider when designing a pole for a particular use is the compressive and tensile strength and modulus that the pole must possess to provide a desired degree of bending strength and stiffness. On the compressive side of the pole, local buckling resistance may also be needed for the particular pole application. When working with solid materials such as wood or concrete, the desired resistance to buckling is provided by the diameter of the pole and the solid wall construction. When working with metal materials, or other materials that are not provided in the form of a solid pole construction, the resistance to buckling is provided by the local wall thickness of the structure.
In addition to solid wood or concrete poles and poles made from metal having a defined wall thickness (i.e., having an annular construction), it is known to make poles from fiber reinforced materials, such as fiberglass reinforced resin. In one example embodiment, such known fiberglass poles have an annular wall structure formed entirely from fiberglass windings, i.e., that comprise a number of layers formed from fiberglass strips that are impregnated with resin. In such known example, the pole structure comprises an inside diameter wall formed from a plurality of radial windings of resin impregnated fiberglass ribbon, intermediate layers provided in the form of a number of longitudinally positioned resin impregnated fiberglass strips that are individually cut to length and positioned along the length of the pole at various locations and that are disposed over the underlying radial windings, and an outermost layer of resin reinforced fiberglass strips that are also individually cut and positioned longitudinally along a length of the pole and disposed over at least a portion of the underlying intermediate layer.
While the above example demonstrates that is it known to form a pole from fiberglass reinforced resin materials, the reliance on multiple layers of fiberglass reinforced resin material to build the wall thickness needed to provide a desired compressive strength and resistance to buckling results in the production of a pole that is relatively expensive compared to more traditional materials based on the raw material costs.
A composite pole manufactured as described above has the following structural issues: (1) the tensile strength of the longitudinally oriented fibers is very high and imparts the bulk of the strength in the tensile direction; (2) the longitudinally oriented fibers however do not have the same compressive strength as they do tensile strength. The reason for this is that the fibers can reach their full strength in tension because they do not rely on the resin matrix to do so. In compression however, the fibers rely in the resin matrix to not buckle the very small glass fibers in compression. This phenomenon results in tensile strength in the axial oriented fibers that may be 6 to 10 higher than the corresponding compressive strength. (3) In designing for local wall buckling under compression (i.e., the full local wall thickness), the local wall may fail in buckling long before the compressive strength is reached. Therefore, it is desired that an optimum pole design would have equal tensile and compressive strength and the wall thickness would be sufficient to avoid local buckling before compressive crush strength.
Further, the process described above for making a single pole by the sequentially performed steps noted above including cutting and laying individual strips of the fiberglass reinforced resin material forming the intermediate and outer layers, is one that is time consuming and costly from a manufacturing perspective.
Accordingly, it is desired that a pole construction be developed that overcomes some or all or the above noted deficiencies. Namely, it is desired that a pole be constructed from a fiber reinforced resin material in a manner that enables the realization of optimal tensile and compressive strength for providing a desired resistance to bending stress or buckling for accommodating use with popular pole applications such as for carrying utility or power lines. It is further desired that the construction of such a pole be one that is relatively more cost effective to build from a manufacturing and/or raw materials perspective when compared to conventional fiberglass reinforced resin poles. Finally, it is desired that such a pole be manufactured in a manner that does not require the use of exotic machinery, and that can be made from raw materials that are readily available.